1. Field of the Invention
This invention generally relates to a frequency synthesizing and back-end processing circuit and method thereof, and more particularly to a digital frequency synthesizing circuit including a linear feedback shift register, logic units, and an analog or digital mixer and a filter for backend processing.
2. Description of Related Art
In the conventional art, there are several frequency synthesizers such as direct frequency synthesizer, phase lock loop frequency synthesizer, and digital frequency synthesizer. Generally, a frequency synthesizing circuit for backend processing includes a phase lock loop, a mixer, and a filter. FIG. 1 is a conventional back-end processing circuit 100. Referring to FIG. 1, this circuit includes phase lock loop (102, 104, 106), a mixer (108), and a filter (110). The phase lock loop (PLL) includes phase detector 102, a low pass filter (LPF) 104, and a voltage controlled oscillator (VCO) 106. The PLL up-converts the received signal to a RF signal. The mixer 108 mixes the RF signal with the local oscillation frequency. The filter 110 then filters the mixed signal to obtain the synthesized frequency.
However, the conventional art has some drawbacks. First, the PLL is not a low power consuming device because it requires a relatively long period to transmit the precise frequency. Further, generally an analog circuit designer has to make a choice between signal stability and phase locking time, which usually causes a lose-lose situation. In addition, an analog circuit usually has fabrication migration issue and the quality of the signal will be affected due to the choice of the loop phase bandwidth of the analog PLL.
Hence, the present invention provides a fully digital frequency synthesizer and a more flexible back-end processing circuit, by using a digital circuit with a simplified analog circuit, to avoid the issue generated by the analog PLL.